There are downhole tools that have integrated pressurized gas chambers that are generally used to offset hydrostatic pressure from a fluid column in a control line that extends from the surface to thousands of meters into the wellbore. Such chambers are illustrated in a subsurface safety valve (SSSV) in U.S. Pat. No. 6,109,351. These downhole tools, when assembled for service are charged with pressure and can sometimes sit in storage for extended periods of time before being deployed downhole. Due to the passage of time from initial charging to actual use, there is uncertainty as to whether the charge is actually still in the chamber or concern that it might not have been charged at all upon assembly. Due to the nature of the service of such tools, they do not feature external gauges to indicate internal pressure because of the risk that such devices may break off during run in. As a result, the tools need to be picked up and mounted in a test fixture and function tested to determine that the gas chamber or chambers are properly charged with the required pressure.
Once the tool is in the hole, there again has been no way to determine if the pressure in the chambers is being retained or if it is slowly dissipating or gone. There are times when a SSSV closes and refuses to open downhole, leaving doubt as to what among several causes could be the reason for such an event.
Accordingly, it is advantageous to know whether there is a charge in a gas chamber of a downhole tool before it goes into service and after it is in service as a diagnostic tool for a malfunction or an early warning tool of an eventual failure. The present invention addresses this need and one application of the invention in a SSSV is described in the description of the preferred embodiment and associated drawing. Those skilled in the art will appreciate that the full scope of the invention is determined by the claims attached to the application.